Differential expression patterns of GABA transporters (GAT1-3) in the rat olfactory bulb.

The localization of GABA transporters 1-3 (GAT1-3) was investigated in the rat olfactory bulb by using in situ hybridization and immunohistochemistry. In the glomerular and the internal granular layers, GAT1 mRNA was expressed in most of periglomerular and granule cells, which are known to be GABAergic. In addition, we compared GAT1 mRNA expression with that of glutamic acid decarboxylase67 (GAD67) mRNA. The expressions were very similar in these two layers, indicating that GAT1 mRNA is mainly expressed in GABAergic neurons. However, in the external plexiform and the olfactory nerve layers, we observed more GAT1 mRNA-positive cells than GAD67 ones, suggesting that GAT1 mRNA is also expressed in non-GABAergic neurons and in glial cells. GAT3 mRNA expression was observed in small glial-like cells which might be involved in GABAergic neurotransmission throughout the olfactory bulb. This was confirmed by double-immunostaining studies which showed the expression of both GAT3 and glial fibrillary acidic protein (GFAP) mRNAs in astrocytes. Moreover, GAT2 mRNA was expressed only in the ependyma and arachnoid. These findings suggest that the expression patterns of GABA transporters differ with the type of cells in the rat olfactory bulb where GAT1 and GAT3 may play an imporatant role in GABA-mediated transmission, such as lateral inhibition.